In wireless, or cellular, communication networks, it is important that the downlink and uplink frame timing be synchronized between a base station and mobile devices served by the base station. The mobile devices connected to the base station use the same transmit and receive frequencies. To ensure that there is no interference between the mobile devices, the mobile devices are assigned time slots or sub-channel frequencies depending on the type of multiplexing (e.g., Time Division Duplex (TDD) or Frequency Division Duplex (FDD)). In either case, frame timing must be precisely maintained over radio links between the base station and the mobile devices.
As illustrated in FIG. 1, according to the Common Public Radio Interface (CPRI) specification, a base station 10 includes a radio equipment controller (REC) 12 and a radio equipment (RE) 14. The REC 12 is typically connected to the RE 14 via one or more cables, such as one or more fiber cables. The REC 12 includes radio functions in the digital baseband domain, whereas the RE 14 includes analog radio frequency functions. A generic interface between the REC 12 and the RE 14, which is referred to herein as a CPRI interface, enables communication between the REC 12 and the RE 14.
In order to maintain precise frame timing, a number of delays are determined. Specifically, as shown in FIG. 1, these delays include:                T12: Cable delay (T12) is a delay between an output interface (R1) of the REC 12 and an input interface (R2) of the RE 14 due to a cable connecting the output interface (R1) of the REC 12 to the input interface (R2) of the RE 14,        TOFFSET: In-equipment delay (TOFFSET) is an internal delay of the RE 14 from the input interface (R2) of the RE 14 to an output interface (R3) of the RE 14,        T34: Cable delay (T34) is a delay from the output interface (R3) of the RE 14 to an input interface (R4) of the REC 12 due to a cable connecting the output interface (R3) of the RE 14 to the input interface (R4) of the REC 12,        T14: Total round-trip delay (T14) that is the sum of T12, TOFFSET, and T34,        T2a: In-equipment downlink delay (T2a) is an internal delay of the RE 14 from the input interface (R2) of the RE 14 to a transmit port (TX) or antenna of the RE 14, and        Ta3: In-equipment uplink delay (Ta3) is an internal delay of the RE 14 from a receive port (RX) or antenna of the RE 14 to the output interface (R3) of the RE 14.        
In operation, the RE 14 estimates the in-equipment delay (TOFFSET), the in-equipment downlink delay (T2a), and the in-equipment uplink delay (Ta3) and reports those estimates to the REC 12. The cable delays (T12 and T34) are then determined by the REC 12 using a synchronization process. For the synchronization process, the REC 12 transmits a sync byte, which is referred to as a K28.5 sync byte in the CPRI specification, from the output interface (R1) of the REC 12 to the input interface (R2) of the RE 14. The RE 14 then passes the sync byte from the input interface (R2) of the RE 14 to the output interface (R3) of the RE 14 such that the sync byte is looped-back to the REC 12. Using the sync byte, the REC 12 measures the total round-trip delay (T14), which is the amount of time between a time at which the sync byte was sent from the output interface (R1) of the REC 12 and a time at which the sync byte was received at the input interface (R4) of the REC 12. Then, the REC 12 computes the cable delays (T12 and T34) as:
            T      12        =                  T        34            =                        (                                    T              14                        -                          T              OFFSET                                )                2              ,where T14 is the total round-trip delay measured by the REC 12 using the sync byte.
In some implementations, strict requirements are placed on the maximum in-equipment downlink delay (T2a) that can be reported to the REC 12. An issue arises when the actual in-equipment downlink delay (T2a) is greater than the maximum allowable in-equipment downlink delay (T2a) that can be reported to the REC 12. As such, there is a need for systems and methods for addressing this issue without totally redesigning the RE 14.